1. Field of the Invention
The present invention relates to a process for the preparation of polyisobutenes by cationic polymerization of isobutene or isobutene-containing hydrocarbon streams in the liquid phase in the presence of boron trifluoride as a catalyst, the catalytic activity of the boron trifluoride being eliminated at a desired time by means of a solid deactivator.
2. Description of the Background
High molecular weight polyisobutenes having molecular weights of up to several 100 000 Dalton have long been known, and their preparation is described, for example, in H. Güterbock: Polyisobutylen und Mischpolymerisate, pages 77 to 104, Springer Verlag, Berlin 1959. The highly reactive polyisobutenes, which as a rule have average molecular weights of from 500 to 5 000 Dalton and a high content of terminal double bonds, i.e. vinylidene groups, of, preferably, substantially more than 60 mol %, can be distinguished from these conventional polyisobutenes.
Such highly reactive polyisobutenes are used as intermediates for the preparation of additives for lubricants and fuels, as described, for example, in DE-A 27 02 604. For the preparation of these additives, polyisobutene/maleic anhydride adducts, in particular polyisobutenyl/succinic anhydrides, are first produced by reacting the terminal double bonds of the polyisobutene with maleic anhydride and are then reacted with specific amines to give the final additive. The amount of terminal vinylidene groups in the molecule is one of the most important quality criteria for this type of polyisobutene, since it is mainly the terminal vinylidene groups which react in the adduct formation with maleic anhydride, whereas, depending on their position in the macromolecule, the double bonds present further toward the interior of the macromolecule lead to substantially lower conversion, if any at all, without the addition of halogens.
A concept for the formation of the terminal vinylidene groups and the isomerization of the terminal double bonds in the isobutene macromolecules to give internal double bonds is described, for example, in the article by Puskas et al., J. Polymer Sci.: Symposium No. 56 (1976), 191 or EP-A 628 575. The protonations, deprotonations and rearrangements taking place are equilibrium reactions in which the formation of more highly alkyl-substituted cations is thermodynamically favored. Said reactions are as a rule promoted by traces of acid, in particular by the usual Lewis acid catalyst of the polymerization itself.
A further quality criterion for polyisobutenes having said intended use is their average molecular weight (Mn).
The molecular weight distribution (dispersity, D) of the polyisobutene macromolecules is also a quality criterion for said purpose, since the broader it is, i.e. the greater the scatter of the molecular weights of the polyisobutene macromolecules, frequently the less suitable are the products for the abovementioned purpose.
A person skilled in the art is familiar with a number of processes for the preparation, from isobutene, of reactive polyisobutenes having average molecular weights and dispersities which meet said requirements, in which processes boron trifluoride is used as a catalyst.
Boron trifluoride is used predominantly in the form of donor complexes, in particular with water, alcohols, phenols, carboxylic acids, carboxylic anhydrides, hydrogen fluoride, ethers or mixtures of these compounds. Boron trifluoride, as such or in the form of said complexes, is a catalyst which is extremely effective even at low temperatures (cf. for example DE-A 27 02 604, EP-A 145 235 or EP-A 322 241).
Therefore, if it is intended to terminate the boron trifluoride-catalyzed reaction of the isobutene after a defined conversion and/or a defined selectivity with respect to the macromolecular products has been established, the boron trifluoride must as rule be rapidly and completely deactivated. This deactivation generally comprises decomposing the boron trifluoride with liquid substances or with substances which are soluble in the reaction medium or converting it into other donor complexes in such a way that it is virtually completely withdrawn from the reaction.
In the prior art, for example, alcohols and acetonitrile are used for such complex-forming deactivation of the boron trifluoride with the chemical organic deactivators in the liquid phase (cf. for example DE-A 43 06 384, EP-A 145 235).
A common feature of the known methods for deactivating the boron trifluoride is that they necessitate the elimination or working up of large amounts of liquid which are contaminated with boron trifluoride, its decomposition products and/or organic compounds. When alcohols are used for the deactivation, corrosive hydrogen fluoride may furthermore be formed by reaction with the boron trifluoride, necessitating the use of high-quality and expensive materials for the design of the apparatuses used.
In view of this situation, solutions employing solid deactivators for the boron trifluoride have been developed.
For this purpose, U.S. Pat. No. 4,384,162 proposes withdrawing the boron trifluoride from the reaction solution by means of solid polyvinyl alcohol. However, the process has the disadvantage that it does not meet the requirements for the polymerization of isobutene because, in spite of the adsorption onto the polyvinyl alcohol, the boron trifluoride retains some of its activity and consequently undesired oligomers may be formed subsequently.
U.S. Pat. No. 4,433,197 discloses using silica gel for the same purpose. However, this deactivation, too, is not sufficiently complete. U.S. Pat. No. 4,213,001 even describes the use of boron trifluoride/silica gel as a catalyst for the oligomerization of 1-olefins, which indicates that the boron trifluoride is still catalytically active even after the adsorption.